We will be driving around several countries that are not on most tourists travel lists.
Italy, Croatia, Slovenia, and, maybe, Bosnia, and Montenegro

I can make use of T-Mobile (U.S.) free roaming in Italy and Croatia. I figured I would get a data SIM for my tablet and tether my phone to it via setting up a hot spot. T-Mobile allows you to make calls via wifi free back to U.S.

Is there even one data SIM that would work in Slovenia - and maybe one of the other countries?

Make use of T-M's UMA/wifi free calling from any place in the world with access to wifi. I use an LG4, wife an S7)
A/o Oct 20, 2013 no need for intl prepaid as T-Mobile U.S. includes voice roaming at 20¢/min (in and out)., unlimited text (in and out), and unlimited EDGE data in 140+ countries.

Another very good option is Orange Mundo from Spain, they have deal 1€ for 100 MB Internet in all EU, activates automatically, which means when you use internet it charges 1€ for each 100MB example you use 1GB so you will be charged 10€ only. it is valid in all EU countries but not included ex-Yugoslavian countries

you can look for these sim cards on ebay if you don't have anyone in Spain or Germany who will send it to you

Generally, I agree with lover's 08 suggestions. But I think it's no use to get an international SIM card for your intineary.

You can use your T-Mobile US inten. option in Italy and Croatia. But you will be throttled in speed of course.
On most plans, it will be only 128 kbps. So better check, if this is enough speed for your VoIP app. I got my doubts.

Generally, I agree with lover's 08 suggestions. But I think it's no use to get an international SIM card for your intineary.

You can use your T-Mobile US inten. option in Italy and Croatia. But you will be throttled in speed of course.
On most plans, it will be only 128 kbps. So better check, if this is enough speed for your VoIP app. I got my doubts.

So don't worry, but be sure that your device covers European frequencies.

Thanks.
1. The throttled data speed from TM ranges a solid 110-112Kbps in almost all the countries we've been in. So solid, used Google GPS navigation for 8 days driving around Romania last August w.o. one glitch. Have used it in 9 countries so far. China, Hong Kong, Germany (just at airport), Suisse, Austria, Hungary, Romania, and about 30-45 minutes on the Ukraine system when near the border w. Romania. Oh, and Costa Rica w. lots of GPS usage.
Wife's FaceBook works fine.
2. We have decided to skip Bosnia. If we hit Montenegro, will be only for a short time, so nothing will be needed there. Main need is for Slovenia.
3. I'll check out the Slovenia link.
4. Don't need voice "app." When in Italy and Croatia will make use of wifi for free calls via TM roaming, or, just pay the 20¢/min. for cell tower calls. What I hope to use the data SIM for in the tablet is to set up a "hot spot" with the tablet (a wifi hot spot) and connect the phone to it for the free wifi features (voice, data, text) in Slovenia. I don't mind paying 20¢/min. for calls. I would mind paying the $2.48/min TM charges to roam in Slovenia, 50¢ for outgoing SMS, and $15/Mb for data.

All phones now sold here seem to work on at least the basic freq. (800/900/1800/1900) Anything above the lowest level phones operate on multiple other freq. as well. Right now I have an S4 and wife, S5 (Samsung) "high" end phones. Old tablet runs Android OS4.0.4 and operates at GSM: 850/900/1800/1900 MHz; UMTS: 1700/2100, 1900, 850; HSDPA+ 21M, HSUPA 5.76M.

Question - What data speed does one need for decent voice connection?

Thanks again.

mike

Make use of T-M's UMA/wifi free calling from any place in the world with access to wifi. I use an LG4, wife an S7)
A/o Oct 20, 2013 no need for intl prepaid as T-Mobile U.S. includes voice roaming at 20¢/min (in and out)., unlimited text (in and out), and unlimited EDGE data in 140+ countries.